Recovery of precious metals from refractory low-grade ores

ABSTRACT

Low-grade refractory gold ores, which may also contain silver and other metal values are treated by partial roasting of concentrate to remove controlled amounts of sulfur and carbon, then oxygen pressure leached to oxidize further amounts of sulfur and carbon and to dissolve base metals and a portion of any silver present, and the residue is then cyanided to dissolve gold and remaining silver which are then recovered.

This application is continuation-in-part of our copending applicationSer. No. 701,258 filed Feb. 13, 1985, now abandoned.

This invention relates to the recovery of gold and silver from low-graderefractory ores in which the presence of base metal sulfides, as well asthe organic carbon, renders the conventional cyanidation processineffective.

BACKGROUND OF THE INVENTION AND THE PRIOR ART

Most of the primary gold and silver produced today is recovered by aprocess which involves cyanidation of the ore to dissolve precious metalvalues, followed by recovery of dissolved values from solution usingzinc-dust cementation or activated carbon adsorption processes.Cyanidation is a simple and relatively inexpensive process that worksvery effectively on free milling ores. In these ores, gold and silverare present in free form in a matrix of oxide gangue.

However, in many other ores, gold and silver are finely disseminated insulfide minerals such as pyrite, pyrrhotite, arsenopyrite, sphalerite,etc., which adversely affect the efficiency of the cyanidation process.The effect is both physical and chemical in nature. Physically, cyanideions cannot reach gold particles entrapped inside dense sulfideparticles. Chemically, sulfide minerals consume oxygen and cyanide,thereby retarding gold dissolution.

There are also some oxide ores in which organic carbon is present whichadversely affects the extraction of gold. In this case, although thegold is dissolved by cyanide solution, it is prematurely removed fromthe solution by adsorption on the organic carbon.

Such ores, which due to the presence of sulfide minerals or organiccarbon or the like are not amenable to conventional cyanidationtreatment, are called refractory ores. As the known reserves of freemilling gold ores are being depleted, attention is being focused ondeveloping efficient processes to recover gold and silver fromrefractory ores, many of which are low-grade as well. The presentinvention is aimed at providing such a process.

Processes have been developed in the past to treat sulfide ores or orescontaining organic carbon. These processes, briefly described below,generally involve an oxidation pretreatment step prior to cyanidation.When any such process is applied to a low-grade ore i.e., an orecontaining only up to about 3 to 5 ppm gold and/or up to about 25 to 50ppm silver, containing both sulfides and carbon, the dissolution of goldand silver in cyanide solution is still relatively low. The presentprocess significantly enhances the recovery of gold and silver fromlow-grade carbonaceous sulfide ores.

There are several reasons for poor gold and silver extractions duringthe cyanidation of the sulfide ores:

1. The gold and silver values may be trapped inside the sulfide mineralgrains either in solid solution or at extremely fine dissemination. Thuscyanide solution cannot reach them even at very fine grinding.

2. The sulfide minerals react in the system consuming valuable reagents,as well as depriving the solution of available oxygen, leading to slowgold dissolution rates.

3. The presence of sulfide minerals also causes electrochemicalpassivation of the gold, thus stopping the leach process altogether.

In order to overcome these problems, the sulfide minerals have to beoxidized to liberate the precious metal values and render them amenableto cyanidation.

Roasting of the sulfide ore or a sulfide concentrate is a commonpractice to accomplish this oxidation. Commercial operations that roastrefractory sulfide ores or concentrates and then cyanide the calcine(roaster product) to recover the gold are known in the art. Referencecan be made, for example, to publications such as F. W. McQuiston, Jr.and R. S. Shoemaker, Gold and Silver Cyanidation Plant Practice,Monograph, The American Institute of Mining, Metallurgical and PetroleumEngineers, Inc., New York 1975; and M. C. Jha and M. J. Kramer,"Recovery of Gold from Arsenical Ores", in Precious Metals: Mining,Extraction, and Processing, V. Kudryk, et al. (editors), TheMetallurgical Society of AIME, Warrendale, Pa., 1984, pp 337-365. Whilesingle- and multiple-hearth roasters were commonly used before the1950's, fluidized-bed roasters are almost universally used now.Depending upon the sulfur and/or arsenic contents of the feed, which inmost cases is a flotation concentrate, different roasting temperaturesin the range of 450° to 700° C. have been used by different plants. Someplants, particularly those treating high arsenic feed, use a two-stageroasting practice. Less than stoichiometric quantities of air (requiredto oxidize all of the sulfur and arsenic to their oxides) is used in thefirst stage, where most of the arsenic and about half of the sulfur areoxidized to volatile As₂ O₃ and SO₂ gas, respectively. The remainingsulfur is burned in the second stage with an excess of air.

A major technical problem associated with roasting is the sintering ofparticles, entrapping gold and silver, which diminishes the recovery ofthese precious metals in the subsequent cyanidation step. This problemis more severe when some other base metal sulfides are also presentalong with the pyrite. The oxides of these metals combine with the ironoxide to form ferrite compounds, which have lower melting points. Thegold and silver entrapped in these ferrite sinters are not amenable tocyanidation.

As an alternative oxidation process to avoid problems associated withsintering of particles during roasting, autoclave oxidation of sulfideconcentrates was recommended in U.S. Pat. No. 2,777,764. Severalexamples cited in this patent demonstrated that autoclave oxidation,followed by cyanidation, resulted in higher gold extractions thancyanidation of sulfide concentrates or roasted calcines. A majorshortcoming of this autoclaving process is that expensive oxygen is usedrather than the air which is used in roasting. Moreover, the amount ofoxygen required is considerably more than in the roasting because sulfuris oxidized to sulfate form rather than to sulfur oxide. Furthermore,the sulfate solution has to be neutralized prior to its disposal. Thus,the autoclave oxidation process, while attractive from the gold recoverypoint of view, has not been pursued because of the expensive oxygenrequirement. This expense is directly related to the sulfur content ofthe feed, and the process will be least attractive for sulfide materialscontaining high amounts of sulfur, and low gold and silver values.

Turning to the treatment of carbonaceous ores, aqueous oxidation hasbeen suggested as a pretreatment. Thus, U.S. Pat. No. 3,574,600describes the use of either an air-ozone mixture or an alkaline sodiumor calcium hypochlorite solution to treat the ore prior to cyanidation.A two-stage oxidation process, using air or oxygen in the first stageand chlorine in the second stage, is described in U.S. Pat. No.4,259,107. The incentive for using air or oxygen in the first stage wasto decrease the chlorine requirements. It is worth noting that the oresample used in the first case contained no sulfur, only 0.35 percentorganic carbon and about 7 ppm gold. The ore sample used in the secondcase contained a small amount of sulfur (about 0.5 percent), a littlehigher (0.7 percent) organic carbon, and considerably higher (14 ppm)gold. Chlorination is also suggested as a pretreatment step in U.S. Pat.No. 4,289,532.

It is obvious that none of these processes can be applied to low-gradecarbonaceous sulfide ores that may typically contain only about 3 to 4ppm (i.e., about 0.09 to about 0.13 oz./ton) gold and 20 to 30 ppmsilver with 0.5 to 1 percent organic carbon and 5 to 10 percent sulfur.The concentrates produced from such ores may contain about 10 to 15 ppmgold and 70 to 100 ppm silver with 1 to 3 percent organic carbon and 20to 30 percent sulfur. Aqueous oxidation treatment of such ores orconcentrates will involve an excessive requirement of oxygen and/orchlorine.

Thus, one is left with no choice but to adopt a roasting pretreatmentprior to cyanidation. As explained before and illustrated by exampleslater, the recovery of gold following this roasting and cyanidationprocess is limited to about the 70 to 80 percent range due to theformation of ferrites during roasting. Silver recoveries can vary from afew percent to about 30 percent.

We have now developed a process in which a combination of roasting andautoclave oxidation is used prior to cyanidation, resulting in 90 to 95percent or even higher dissolution of gold in the cyanidation step.Silver dissolution is also significantly improved.

OBJECTIVES OF THE INVENTION

The prime objective of the invention is to provide a process forenhanced recovery of gold and silver from carbonaceous sulfide ores,particularly ores of low grade. Another objective is to providepretreatment processes to oxidize sulfur and carbon present in the orein such a way that gold and silver values are not entrapped inrefractory ferrite compounds and are thus amenable to cyanidation. Stillanother objective is to accomplish the oxidation process withoutconsuming excessive amounts of expensive oxygen in the autoclaveoxidation step, while solubilizing some silver and essentially all ofthe base metal values in the autoclave liquor from which it can berecovered.

DESCRIPTION OF THE DRAWING

The drawing consists of a flowsheet setting forth the preferred methodfor carrying out the invention.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the invention the refractory gold- and/orsilver-bearing sulfide ore, which usually also is carbonaceous, iscrushed, ground and concentrated. Flotation or gravity separationmethods can be used. The concentrate is then partially roasted tooxidize a major (i.e., more than 50%) proportion of the sulfur but onlya minor (i.e., less than 50%) proportion of the carbon in theconcentrate. The calcine is then autoclaved in the presence of oxygen tooxidize the remaining sulfide sulfur and organic carbon, to decomposeany ferrites in the calcine and dissolve base metals and some silver,which can be recovered from the leach liquor. The autoclave residue,after washing, is subjected to the standard cyanidation process. Golddissolution is found to be 90% to 95% or higher while most of the silverpresent in the feed is also dissolved. The dissolved gold and silvervalues can be recovered from the cyanide solution using standardpractices such as zinc-dust cementation or activated carbon adsorption.

Desirably, the roasting temperatures are in the range of about 600° to650° C. During roasting, less than stoichiometric (theoretical amountneeded to oxidize all the sulfur and carbon present in the feed)quantities of oxygen, as air, is used. This is done by controlling theair flow rate in relation to sulfide feed rate to the roaster. About 65to 80 percent of the sulfur present in the feed is removed as SO₂ gas.The carbon oxidation is limited to about 15 to 30 percent.

Fluid bed roasters, rotary kilns, etc., can be employed in the roastingstep. Sulfur dioxide generated during roasting can be converted tosulfuric acid or can be scrubbed with alkali and disposed of suitably.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in conjunction with the drawing inwhich reference character 11 indicates low-grade carbonaceous sulfideore which is fed to the process for crushing 12, grinding 13, flotation14 to which reagents 15 are fed and from which tailings 16 are rejected,as is or after cyanidation to recover any gold or silver values presentin them. Concentrate 17 is fed to the partial roasting operation 18 towhich air 19 is fed and from which SO₂ 20 is led to the sulfuric acidplant (not shown) and calcine 21 is delivered to autoclave oxidationstep 22 to which oxygen 23 is supplied. The autoclave product is led tosolid-liquid separation and washing step 24 which is supplied with water25. The solution resulting is taken to recovery step 26 while theresidue is fed to cyanidation 27 which is supplied with reagents 28 andfrom which rejected tailings 29 flow. The solution is taken to recoverystep 30.

While the process of this invention can be applied to any gold- orsilver-bearing sulfide ore, it is particularly attractive for treatinglow-grade carbonaceous sulfide ores of gold found in several parts ofthe world. Typical analysis of this type of ore is presented in Table 1,below.

                  TABLE 1                                                         ______________________________________                                        Typical Analysis of Low-Grade Carbonaceous                                    Sulfide Ore Used in Test Work                                                 Element         Analysis                                                      ______________________________________                                        Gold            3          ppm                                                Silver          20         ppm                                                Iron            6          percent                                            Zinc            1          percent                                            Sulfur          7          percent                                            Organic Carbon  0.7        percent                                            ______________________________________                                    

When samples of this ore were subjected to the standard cyanidationtest, only about 25 percent of the gold and about 10 percent of thesilver could be dissolved, as shown by Example 1. This refractory natureof the ore is not surprising in view of the high sulfur and carboncontents of the ore.

Concentrate Preparation

It is necessary to oxidize the sulfur and carbon contents in order toenhance the recovery of gold and silver during subsequent cyanidation.In order to keep the size of the oxidation and cyanidation plants small,it is prudent, although not necessary, to physically beneficiate suchlow-grade ores and recover most of the precious metal values in anupgraded sulfide concentrate. The increased sulfur and carbon contentsalso help in making the oxidation process autogenous in terms of energyrequirements.

When samples of this particular ore were crushed, ground, and floatedwith standard sulfide flotation reagents, about 85 percent of thecontained gold and silver values were recovered in a sulfide concentrateweighing about 25 to 30 percent of the original feed. Table 2 lists thetypical range of composition for such concentrates.

                  TABLE 2                                                         ______________________________________                                        Sulfide Concentrates Produced by Flotation                                    Element        Analysis Range                                                 ______________________________________                                        Gold           9 to 11      ppm                                               Silver         50 to 80     ppm                                               Iron           19 to 25     percent                                           Zinc           1.1 to 2.8   percent                                           Sulfur         22 to 29     percent                                           Organic Carbon 2 to 3       percent                                           ______________________________________                                    

Cyanidation of such flotation concentrates, as shown by Example 2,resulted in only about 30 percent dissolution of gold and silver.Conventional oxidation pretreatment processes using oxidants such aschlorine, air, oxygen, etc., improved the extractions only marginally.Increasing the fineness of the grind did not improve the extractions."Preg-robbing" of gold (premature precipitation on carbon) in particularand that of silver to a lesser degree, was observed. Reducing thecyanidation time to minimize "preg-robbing" improved the extractions by10 percent or less. This indicates that most of the precious metals arepresent in the grains of sulfide minerals and are, therefore, notamenable to cyanidation.

Roasting

To oxidize the sulfur and carbon present in the concentrates, andthereby make them amenable to cyanidation, flotation concentrates wereroasted in a 15-inch fluidized-bed furnace at temperatures in the rangeof 600° to 700° C. The typical composition range of the resultingcalcines is presented in Table 3. There is some upgrading of gold andsilver contents due to weight loss associated with oxidation of sulfurand carbon.

                  TABLE 3                                                         ______________________________________                                        Typical Composition Range of Calcines Produced                                by Fluidized-Bed Roasting of Sulfide Concentrates                             Element       Analysis                                                        ______________________________________                                        Gold          11 to 14       ppm                                              Silver        100 to 150     ppm                                              Iron          24 to 26       percent                                          Zinc          3 to 4         percent                                          Total Sulfur  1.5 to 2.3     percent                                          Organic Carbon                                                                              0.2 to 1.4     percent                                          ______________________________________                                    

When calcine samples I and II were subjected to standard cyanidationtests, the gold extractions were 62 and 69 percent, respectively, asshown by Example 3. Treating the calcine samples with hot sulfuric acid,followed by washing them with hot water prior to cyanidation, improvedthe gold dissolution further to 70 percent and 85 percent, respectively,as shown by Example 4. The silver extractions were 38 and 50 percent,respectively, from the first sample (untreated and treated) and only 27percent from the second sample, regardless of the treatment. The secondsample, containing lower sulfur and carbon contents, represents morecomplete roasting. Poor silver extraction from this sample would suggestfixation of silver in ferrites, thereby impeding its dissolution incyanide solution.

It should be mentioned here that while gold and silver are both preciousmetals and it is desirable to improve recovery of both metals, gold isconsiderably more precious than silver. Thus, an improvement of 1percent in gold extraction may be as important as an improvement ofseveral percent in silver extraction.

The results presented so far (Examples 1 to 4) indicate that carefulimplementation of the state-of-the-art technology (flotation, roasting,acid treatment, and water washing prior to cyanidation) may result in 70to 80 percent gold dissolution from these low-grade gold concentratescontaining 10 to 14 ppm gold. In view of the recent past and projectedfuture gold prices, it is desirable to improve the gold dissolution to90 to 95 percent or more, and it is possible to do so by the process ofthis invention.

Partial Roasting - Autoclaving

One of the reasons for incomplete dissolution of gold and silver fromthe roasted calcines is the physical and/or chemical inclusion of thesemetals in the ferrite compounds formed during roasting. Formation ofsuch ferrites is more likely when the concentrate contains base metalsulfides, such as zinc sulfide in the present case. Moreover, selectiveflotation of a sphalerite (zinc sulfide) concentrate indicated thatsignificant amounts of gold and silver were present in this mineral.Therefore, it is very likely that gold and silver present in thismineral will be converted to inert ferrites during roasting and will notbe amenable to cyanidation. The problem becomes even more serious if ahigher temperature (e.g. temperatures of about 700° or higher) is usedfor roasting. Unfortunately, for concentrates containing organic carbon,such high temperatures are unavoidable in order to oxidize sulfur andcarbon completely.

However, if only a part of the sulfur and carbon are oxidized in apartial roasting operation, higher temperatures and stronger oxidizingconditions can be avoided. Such an operation would eliminate or minimizeinclusion of gold and silver in a refractory ferrite structure. Next,aqueous oxidation in an autoclave could be used not only to oxidize theremaining sulfur and carbon but also to dissolve the zinc and decomposeany ferrite present in the partially roasted calcine. It is also likelythat hematite formed under autoclave conditions would be more porousthan the hematite formed during high temperature roasting, and thisshould again facilitate the dissolution of gold and silver in asubsequent cyanidation operation.

To illustrate the benefits of combining partial roasting and autoclavingas pretreatment steps prior to cyanidation, several batches of flotationconcentrates were partially roasted in fluidized-bed furnaces of 4 to12-inch diameter at temperatures of 550° to 650° C. with less thanstoichiometric amounts of air supplied to the roaster. The compositionrange of resulting calcines is presented in Table 4. There is a smallupgrading of gold and silver contents due to removal of about 70 percentof the sulfur in the partial roasting step. Due to lower temperature andoxygen deficiency used in the roasting step, carbon elimination wasunder 25 percent.

                  TABLE 4                                                         ______________________________________                                        Composition Range of Partially Roasted Calcines                               Element     Analysis                                                          ______________________________________                                        Gold        9.7 to 11.3     ppm                                               Silver      55 to 86        ppm                                               Iron        1.9 to 27       percent                                           Zinc        1.2 to 3.1      percent                                           Sulfur      6.3 to 8.1      percent                                           Carbon      2.3 to 2.8      percent                                           ______________________________________                                    

Cyanidation of partially roasted calcines resulted in about 60 percentgold extraction and lower silver extractions as shown by Example 5.

Next, the partially roasted calcines were subjected to aqueous oxidationin an autoclave. The residue from the autoclave was washed with waterand then cyanided under standard conditions. Gold extractions of 90 to95 percent or higher were obtained, depending upon the autoclavingconditions.

It should be remembered that the gold extraction numbers mentionedhereinafter refer to the percent of gold dissolved during thecyanidation step. No dissolution of gold was detected in the autoclavingstep. However, for silver, the numbers reported are for overalldissolution, some taking place in the autoclave itself and the rest inthe cyanidation step. The silver dissolved in the autoclave, along withzinc or other base metals such as cobalt, nickel, copper, etc., whichdissolve completely under autoclave oxidation conditions, can berecovered from the autoclave liquor using standard practices, forexample, sulfide precipitation.

Effect of Temperature: Since the success of the process depends upondecomposing the ferrite and transforming it, pyrrhotite, and pyrite toporous hematite, the temperature of the autoclave is an importantvariable. Higher temperatures improve the kinetics of transformation butmake the construction and operation of the autoclave more expensive.Thus, a balance has to be struck. A temperature of 200° to 220° C. gaveexcellent results, about 95 percent gold extraction, as shown by Example6. The process can be effectively carried out at lower temperatures, say150° to 180° C., but a longer retention time would be needed. Similarly,the autoclave can be operated at a higher temperature, 250° or 270° C.,to reduce the retention time, but this would involve extra capital andoperating costs.

Oxygen Pressure and Pulp Density: The oxygen overpressure is anotherimportant variable affecting both the rate of conversion and the cost ofthe autoclave construction and operation. Again, a higher oxygenpressure would be desirable from the process kinetics point of view, butit would increase the cost of the autoclave and compressor. The effectof oxygen pressure would also be a function of pulp density, which inturn determines the oxygen demand rate. As can be seen from the resultspresented in Example 7, an oxygen overpressure of 50 psi (measured overand above the steam pressure at the autoclave temperature) was adequatefor 10 percent solids in the pulp, but an overpressure of 100 psi wasneeded at 30 percent solids, which would be about the maximum pulpdensity in a continuous commercial operation.

Other Variables: There are other autoclave process variables such asintensity of agitation and rate of gas bleeding that will affect theresults of the process. However, the effect of these variables isinfluenced by the geometry of the autoclave and the mode of operation(i.e., batch vs. continuous). Thus, specific ranges for these variablescannot be specified. The basic idea is to provide enough agitation notonly to insure off-bottom suspension of all the solid particles but alsoto provide good mass transfer rate for oxygen from the gas phase to thesolids/solution interface where it is consumed to oxidize sulfur andcarbon. A certain amount of gas bleeding should be provided to removethe inert carbon dioxide gas from the system. The extent of bleedingprimarily depends upon the carbon content of the feed. Excellent resultsare obtained even when carbon is not oxidized completely. It seemspossible that autoclaving may deactivate the organic carbon and therebycounter its bad effect during subsequent cyanidation.

Versatilitv of the Process: To determine and demonstrate the versatilityof this novel process, aqueous oxidation treatment in an autoclave wasapplied to various samples of partially roasted calcines produced fromdifferent flotation concentrates under different roasting conditions.The results are presented in Example 8. Autoclaving was performed for 4hours on pulps containing 10 to 30 percent solids at a temperature of220° C. and an oxygen overpressure of 100 psi. The residues were washedwith water and then cyanided under standard conditions. Gold extractionof over 93 percent was obtained in each case. The silver extraction wasvariable. High extractions of 70 to 88 percent were observed for silverat the lower pulp density.

To further demonstrate the versatility of the process, samples of ore,concentrate, and totally roasted calcine were subjected to standardautoclaving and cyanidation tests. As the data presented in Example 9indicate, over 90 percent gold extraction was obtained in each case.

However, due to the large oxygen demand (as detailed in Example 9),direct autoclaving of ore or concentrate may not prove economical. Sinceall the sulfur present in the ore is oxidized to the sulfate form in theautoclave, not only are the credits for sulfuric acid lost but the neuralization costs for the autoclave liquor may also be substantial. Onthe other hand, treatment of totally roasted calcine may not be veryattractive, both because of larger roaster capacities needed in thiscase and poor silver extractions from such calcines even afterautoclaving.

The combination of partial roasting of the concentrate, followed by itsoxidation in an autoclave, results in 90 percent or higher goldextractions in subsequent cyanidation steps. The gold and silver can berecovered from the cyanide solution by standard processes.

EXAMPLE 1

In this, as well as in the following examples, standard cyanidationtests involved leaching of 60 g dry solid feed with 180 ml of 1.5 g/LNaCN solution in a stirred glass reactor for 24 hours at roomtemperature. Lime slurry was added to control the pH at 11. Air wassparged to insure a reduction potential (Eh) of at least -100 mv,measured against a standard calomel electrode. Samples of solution wereanalyzed for free cyanide concentration, and concentrated NaCN solutionwas added as required to maintain the NaCN strength at about 1.5 g/Lthroughout cyanidation. The tailings (residue) were washed, dried,weighed, and fire assayed for gold and silver. The feed and tail assayswere used to calculate the percent dissolution or extraction of gold andsilver.

A sample of low-grade carbonaceous sulfide ore analyzed 3.9 ppm gold, 20ppm silver, 7.16 percent iron, 1.0 percent zinc, 7.4 percent sulfur, and0.76 percent organic carbon. The ore sample was subjected to a standardcyanidation test as described above. Only 22 percent of the gold and 9percent of the silver dissolved. Another cyanidation test was performedafter regrinding the feed sample. The gold and silver dissolutionimproved only to 24 and 15 percent, respectively. The chemical analysisof the cyanide solution samples indicated a decline in gold and silverconcentrations from the 6-hour to 24-hour sample. This indicates thatthe organic carbon was causing "preg-robbing" (i.e., prematureprecipitation of gold and silver from the pregnant solution).

EXAMPLE 2

A sample of low-grade carbonaceous sulfide ore of composition similar tothat shown in Table 1, was subjected to crushing, grinding, andflotation treatment to recover gold and silver in a sulfide concentrate.One of the sulfide concentrate samples analyzed 9.7 ppm gold, 76 ppmsilver, 20 percent iron, 2.8 percent zinc, 24 percent sulfur, and 2.5percent organic carbon. When this sample was subjected to standardcyanidation tests (described in Example 1), only about 30 percent of thegold and 28 percent of the silver were dissolved.

EXAMPLE 3

Two samples of calcine produced by roasting flotation concentrates in afluidized-bed furnace were subjected to standard cyanidation tests. Thechemical composition of the calcines and the extent of dissolution ofgold and silver from them during cyanidation are tabulated below. It isseen that roasting pretreatment considerably improved the extraction ofgold from the 20 to 30 percent range (Examples 1 and 2) to the 60 to 70percent range. The extent of silver dissolution remained about the sameor improved only slightly.

    ______________________________________                                                    Calcine Sample I                                                                          Calcine Sample II                                     ______________________________________                                        Composition                                                                   Gold, ppm     12.0          14.2                                              Silver, ppm   104           146                                               Iron, percent 24            26                                                Zinc, percent 3.3           3.8                                               Sulfide Sulfur, percent                                                                     2.35          1.45                                              Organic Carbon, percent                                                                     1.36          0.22                                              Extraction                                                                    Gold          63            69                                                Silver        38            27                                                ______________________________________                                    

EXAMPLE 4

To remove any soluble salts (like sulfates) from the calcine andactivate its surface prior to cyanidation, samples of calcine I and II(described above in Example 3) were treated with dilute sulfuric acid(pH 1) for 1 hour at 90° C. and then washed with hot (90° C.) water.When washed calcines were subjected to standard cyanidation tests, thegold extractions improved to 70 percent for calcine Sample I and to 85percent for calcine Sample II. The silver extraction from Sample Iimproved to 50 percent but that from Sample II remained at 27 percent.

EXAMPLE 5

Two samples of partially roasted calcines were subjected to standardcyanidation tests. The composition of these samples and the extractionof gold and silver from them are tabulated below.

    ______________________________________                                                       Partially Roasted Calcine                                                     Sample III                                                                            Sample IV                                              ______________________________________                                        Composition                                                                   Gold, ppm        11.1      9.9                                                Silver, ppm      55        77                                                 Iron, percent    21        27                                                 Zinc, percent    2.7       1.2                                                Sulfide Sulfur, percent                                                                        7.4       6.4                                                Organic Carbon, percent                                                                        2.3       2.8                                                Extraction, percent                                                           Gold             62        59                                                 Silver           22        46                                                 ______________________________________                                    

It is seen that partial roasting considerably improved the gold andsilver extractions in comparison to extractions from ore or concentrate(Examples 1 and 2). The higher silver extraction from Sample IV could bedue to its higher silver and lower zinc contents in comparison to SampleIII.

EXAMPLE 6

A partially roasted calcine Sample V analyzed 11.3 ppm gold, 86 ppmsilver, 23 percent iron, 3.1 percent zinc, 6.3 percent sulfide sulfur,and 2.6 percent organic carbon. This sample was used in most of theautoclave oxidation work performed to develop the process of thisinvention.

Samples of this partially roasted calcine were pulped with dilute, 25g/L, H₂ SO₄ to a pulp density of 10 percent solids (100 g solid, 900 mldilute H₂ SO₄). The slurry was then heated in a glass lined titaniumautoclave while being vigorously agitated. The autoclave was heated byan electrical jacket with a temperature controller that was activated bythe actual slurry temperature measured through a thermocouple well inthe autoclave. After reaching the desired temperature, reported in thetable below, oxygen gas was added to the autoclave to maintain an oxygenoverpressure of 50 psi, as measured on a gauge mounted on the autoclavetop. The reaction was allowed to proceed for 4 hours. After that time,the oxygen gas line was disconnected, agitation was stopped, and theautoclave removed from the electric jacket and cooled under tap water.Any gas pressure remaining in the autoclave was relieved by bleeding,and the contents of the autoclave were poured on a vacuum filter. Theresidue was thoroughly washed, dried, weighed, and a sample wasanalyzed. A 60 g portion of the residue was subjected to the standardcyanidation test as described in Example 1.

A series of four tests was performed with the autoclave temperature setat 150°, 180°, 200°, and 220° C., respectively. Excellent gold andsilver extractions (above 95 percent and 85 percent, respectively) wereobtained when the temperature was 200° or 220° C., as shown by the datapresented in the following table. It should be mentioned here thatpractically no gold dissolved in the autoclave. However, as the data inthe table indicate, a variable amount of silver dissolved in theautoclave. The silver extractions reported in the table were computedfor overall dissolution of silver from autoclave feed to cyanidationtailings. The data also indicate that almost all the zinc dissolved inthe autoclave step. Other base metals, like copper, cobalt, etc., wouldhave behaved the same way. The data also indicate that at lowertemperatures (150° or 180° C.), the sulfur oxidation process was notcomplete and this aversely affected the extent of gold and silverdissolution.

    ______________________________________                                        Test Number      1       2       3     4                                      ______________________________________                                        Autoclave Temperature, °C.                                                              150     180     200   220                                    Oxygen Consumption                                                                             --      191     189   213                                    (lb/ton of feed)                                                              Autoclave Product Analysis                                                    Gold, ppm        11.1    11.7    12.0  12.0                                   Silver, ppm      88      82      46    30                                     Iron, percent    20.7    24.4    24.8  24.7                                   Zinc, percent    1.6     0.02    0.02  0.03                                   Sulfur, percent  3.9     1.2     0.75  0.58                                   Extraction, percent                                                           Gold             72      88      96    97                                     Silver*          46      88      88    86                                     ______________________________________                                         *Includes extraction in autoclave step.                                  

EXAMPLE 7

In another series of four tests performed at 220° C. the pulp densityand oxygen overpressure were varied. The feed and other autoclavingconditions were the same as described in Example 6. As the resultspresented below indicate, at a low pulp density of 10 percent solids, anoxygen overpressure of 50 psi was adequate. However, at 30 percentsolids, a higher oxygen over pressure of 100 psi gave better results.

    ______________________________________                                        Test Number     1       2        3     4                                      ______________________________________                                        Pulp Density, % Solids                                                                        10      10       30    30                                     Oxygen Overpressure, psi                                                                      50      100      50    100                                    Oxygen Consumption                                                                            213     214      387   387                                    (lb/ton of feed)                                                              Extraction, percent                                                           Gold            97      98       86    94                                     Silver*         86      88       33    52                                     ______________________________________                                         *Includes extraction in autoclave step.                                  

EXAMPLE 8

Samples of partially roasted calcines III, IV, and V (described inExamples 5 and 6) were subjected to autoclave oxidation treatment at thecommon conditions of 220° C. and 100 psi oxygen overpressure. The pulpdensity was 10 or 30 percent solids. As shown by the data presentedbelow, excellent gold extractions were obtained in all cases. Silverextractions varied considerably, reflecting probably both variations infeed composition and roasting conditions.

By lowering the pulp density, the silver extraction was greatlyimproved.

    ______________________________________                                         Test Number  1       2       3     4     5                                   Sample Number III     III     IV    V     V                                   ______________________________________                                        Pulp Density  30      10      30    30    10                                  Oxygen Consumption                                                                          334     --      307   310   --                                  (lb/ton of feed)                                                              Extraction, Percent                                                           Gold          96      99      93    94    98                                  Silver*       19      76      29    52    88                                  ______________________________________                                         *Includes extraction in autoclave step.                                  

EXAMPLE 9

A series of tests was performed on a variety of feed materials. Thesematerials were ore, flotation concentrate, and partially and fullyroasted calcine. These were subject to autoclave oxidation for 4 hoursat 220° C. under varying conditions as shown in the following table.Subsequent cyanidation of autoclave residues resulted in over 90 percentgold extraction. The silver extraction was variable, being as high as 92percent from the concentrate.

    ______________________________________                                                  Test Number                                                                   1     2        3      4     5                                                 Feed Material                                                                                Partially  Fully                                                     Con-     Roasted    Roasted                                             Ore   centrate Calcine    Calcine                                   ______________________________________                                        Composition                                                                   Gold, ppm   3.9     10.7     11.1 11.1  14.0                                  Silver, ppm 20      54       55   55    146                                   Iron, percent                                                                             7.6     19       21   21    26                                    Zinc, percent                                                                             1.0     2.4      2.7  2.7   3.8                                   Sulfur, percent                                                                           7.4     22       7.4  7.4   1.45                                  Organic Carbon,                                                                           0.76    2.4      2.3  2.3   0.22                                  percent                                                                       Autoclave Con-                                                                ditions                                                                       Pulp Density                                                                              30      30       10   30    30                                    % Solids                                                                      Oxygen Pressure,                                                                          100     100      50   100   50                                    psi                                                                           Initial H.sub.2 SO.sub.4, g/L                                                             25      0        25   0     100                                   Oxygen Con- 418     871      --   167   94                                    sumption                                                                      (lb/ton of feed)                                                              Extraction, Percent                                                           Gold        94      94       99   96    91                                    Silver*     40      92       76   19    4                                     ______________________________________                                         *Includes extraction in autoclave step.                                  

Oxygen consumption during autoclaving of the "ore" and "concentrate"materials of Example 9 amounted to 418 pounds per ton of ore and 871pounds per ton of concentrate. This corresponds to 244 lb/ton of oresince the concentrate weighed 28 percent of the ore. The oxygenconsumption during autoclaving of partially roasted calcine was 167lb/ton of calcine which corresponds to 42 lb/ton of ore since thecalcine weight was about 25 percent of the ore. While the oxygenconsumption was lowest (94 lb/ton of calcine or about 21 lb/ton of ore),in the case of fully roasted calcine, the silver extraction wasnegligible. Therefore the partially roasted calcine will be thepreferred feed for autoclave treatment. Under suitable conditions asnoted in the table above, up to 76 percent of the silver and essentiallyall of the gold were recovered. The process of the invention thus notonly results in high recoveries of gold and silver from low-graderefractory ores but also provides substantial economies in the use ofoxygen. Similarly the amount of sulfate ions present in the autoclaveliquor either as sulfuric acid or as metal sulfates, are smaller in thecase of partially roasted calcines in comparison to the amounts presentafter treatment of ores or concentrates. A substantial saving in limeslurry required to neutralize the liquors prior to cyanidation isthereby afforded. The saving in oxygen and lime is of particularimportance in operation at a remote location.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

What is claimed is:
 1. The process for treating a refractory low-gradeore containing precious metals from the group consisting of gold andsilver to recover the precious metal content thereof which comprisespartially roasting a concentrate of said ore to oxidize a major portionof sulfur present to sulfur dioxide but to oxidize a minor portion onlyof carbon present therein, subjecting the partially roasted calcine toan oxygen pressure leach to oxidize remaining sulfur and carbon, todecompose any ferrites present and to dissolve base metals and somesilver, and then subjecting the residue from said oxygen pressure leachto cyanidation to dissolve substantially all the precious metals presenttherein.
 2. The process in accordance with claim 1 wherein saidconcentrate is obtained from a low-grade carbonaceous sulfide ore. 3.The process in accordance with claim 2 wherein not more than about 65 to80% of the sulfur and not more than about 15 to 30% of the carbon isremoved during the partial roasting step.
 4. The process in accordancewith claim 2 wherein said concentrate contains about 1% to about 3%, byweight, of organic carbon and about 20% to about 30%, by weight, ofsulfur.
 5. The process in accordance with claim 4 wherein saidconcentrate contains about 8 to about 15 ppm gold and about 50 to about100 ppm silver.
 6. The process in accordance with claim 1 wherein silverdissolved in the oxidation leach liquor is recovered.
 7. The process inaccordance with claim 1 wherein said partial roasting is conducted atabout 550° to about 650° C. in an atmosphere stoichiometricallydeficient in oxygen.
 8. The process in accordance with claim 7 whereinsaid partial roasting is conducted at a temperature in the range ofabout 600° to about 650° C.